1. Field of the Invention
This invention relates to a bipolar semiconductor device for power control.
2. Background Art
Insulated gate bipolar transistors (IGBTs) have been conventionally used as semiconductor devices for power control. In recent years attention has been paid to an anode-shorted IGBT in which a P-type collector layer is partly formed on the lower surface of the chip. In an anode-shorted IGBT, the P-type collector layer is formed only in a partial region of the lower surface of the chip, and an N-type drift layer is exposed to the remaining region of the lower surface of the chip. Furthermore, the P-type collector layer and the N-type drift layer are both connected to the collector electrode. Thus, when the IGBT is turned off, electrons in the N-type drift layer can be rapidly ejected through the collector electrode, enabling the turn-off time to be reduced (see, e.g., JP-A-2006-019556).
However, in such an anode-shorted IGBT, unless the P-type collector layer has a higher potential than the N-type layer, the junction interface between the P-type collector layer and the N-type drift layer does not become a forward junction, and the IGBT fails to be turned on. Thus, unfortunately, the IGBT is difficult to turn on stably.